It is known in the art to routinely inspect steam turbines and generators used for electrical power generation to detect discontinuities. Rotor bore inspections are performed on power generating equipment, such as turbines and generators. These inspections are typically carried out using a variety of convention methods, such as ultrasonic inspection and eddy current inspection. The ultrasonic inspection is employed to inspect areas that are inside the volume of the rotor around the bore, and the eddy current inspection is employed to inspect the bore surface. Other methods, such as visual and magnetic particle examination, have been used successfully to inspect the bore, but some of these methods are only sensitive to discontinuities which intersect or are very near to the bore and then only yield a two-dimensional view of the material and any detected discontinuities.
The objective of these inspections is to determine the presence of flaws or discontinuities inside the volume of the rotor. The results obtained from the inspections are utilized to assess the condition and integrity of the component. The assessment of the component is based on the characteristics of the flaws or discontinuities, such as, for example, size, orientation and location. The greater the precision and accuracy of the inspection technique and data obtained therefrom, the more reliable is the assessment for determining the condition of the component. It is desired to identify discontinuities to preclude progression to a point where they risk the integrity of the component and potential component failure. The consequences of a sudden, catastrophic failure of such a component could be severe. However, there are instances where a large margin of safety is built into the analysis to compensate for the lack of accuracy and resolution of the inspection data. While this approach is safe, it can be inefficient in that the component actually may be able to operate safely for a period of time that extends beyond that identified by a conservative assessment.
A typical manufacturing process for turbine and generator rotors that are currently in service included a forging process that migrated impurities into the center of the forging. The impurities could be removed by drilling a hole (e.g., bore hole) through the center of the rotor. The size of the bore hole diameter generally relates to the amount and location of impurities near the center line of the rotor. A greater number of impurities resulted in a larger diameter hole being drilled. Although drilling the bore hole is a mechanism for removing most of the impurities, the bore hole can be stressed during operation of the rotor, which can lead to discontinuities in the rotor material. Additionally, other impurities or inclusions that remained outside of the bored area (volume) could grow under service-related stress and could potentially result in a failure.
Conventional ultrasonic inspection methods include applying high-frequency sound waves to the structure being tested using one or more transducers. The transducers typically include piezocrystal elements that are excited by an electrical voltage to induce the ultrasonic waves in the structure. When the ultrasound waves interact with something (e.g., a void, a crack or other defect) having a significant difference in impedance from that of the propagation medium, a portion of the ultrasound is either reflected or diffracted back to the source from which it originated. Detection and quantification of the returned ultrasound pattern is used to determine the characteristics of the reflecting medium. In this method, referred to as rotor boresonic inspection, an automated system is typically used to transport ultrasonic transducers inside the rotor bore hole by some convenient method, and the transducers direct sound, i.e., ultrasonic waves or beams, from the rotor bore surface into the rotor material and toward the rotor bore's outside diameter. The ultrasonic wave can penetrate well into the rotor material, and by collecting, processing, and observing any reflections of the wave which occur within the forging, one can get some idea of the integrity of the material.
In a rotor boresonic inspection, longitudinal ultrasound is directed in a radial direction; shear wave ultrasound is directed at angles from the bore surface clockwise and counter clockwise around the rotor and shear wave ultrasound is also directed in an axial direction along the length of the rotor.
Standard ultrasonic transducers can be used to accomplish rotor boresonic inspection. For example, a shear wave angle beam inspection uses plexiglass wedges to refract the beam to a predetermined fixed angle. If other angles are required during the inspection, the transducers are removed and the wedges changed to obtain the desired refracted angle. Transducer and wedge change out is time consuming and requires numerous wedges and transducers to sweep through several different angles of attack.
Phased array ultrasonic technology generally provides for computer-controlled excitation (e.g., amplitude and delay) of individual elements in a multi-element transducer (as opposed to single-element transducers of conventional ultrasonic inspection). The excitation of piezocomposite elements can generate a focused ultrasonic beam with the potential to modify beam parameters, such as angle, focal distance, and focal point through software. Thus, a computer-controlled beam scanning pattern can be implemented in order to direct or steer the beam to the area of interest and to search for cracks and other discontinuities in the rotor bore.
In a particular known ultrasonic testing system, the transducers are placed on the outside of the turbine or generator rotor and the phased arrays are employed to inspect from the outside of a bore to the inside of the bore. In another known system, water is introduced inside of the rotor and the transducers used to inspect and examine the turbine or generator rotor bores are immersed in the water. Further, in these known systems, each of the transducers are positioned to collect data from a beam emitted from the transducer. The beam is emitted in a single, fixed pre-set/pre-specified direction. The direction of the transducer can be changed by re-positioning the transducer in accordance with the desired direction. For example, the fixed angle wedges previously described herein can be used to re-position the transducer to emit a beam in a different direction.
There remains a need for an improved ultrasonic testing system of turbine and generator rotor bores. The capabilities of known systems have limitations. For example, known systems utilize fixed angle wedges to steer the beam and therefore, re-direction of the beam requires the transducer to be removed and re-positioned at a different location. Further, known systems can include a beam having a fixed focal point such that areas in the near field and far field of the transducer are not inspected. Thus, it is desired to provide an ultrasonic testing system that provides an array of angles to inspect an area of interest (e.g., sectorial scan) and provides the ability to change the focal depth of the transducer.